Overexpression of CD44 in acquired endocrine resistant breast cancer modulates erbB activity and promotes an invasive phenotype.

CTRC-AACR San Antonio Breast Cancer Symposium: 2008 Abstracts Abstract #3027 Background: Acquired endocrine resistance in breast cancer cells is accompanied by altered growth factor receptor signalling and a highly migratory cell phenotype. In tamoxifen-resistant ('TamR') MCF7 cells, our microarray analysis has demonstrated elevated levels of CD44, a transmembrane glycoprotein, known to interact with and modulate the function of a number of growth factor receptors and plays a major role in tumour metastasis. Here, we have explored the role of CD44 as a modulator of erbB family activity in TamR cells and evaluated the expression of CD44 in a series of breast cancer patients on endocrine therapy. Methods: RT-PCR was performed to confirm overexpression of CD44 at the gene level in TamR cells. Western blotting and immunocytochemistry were used to investigate expression of CD44 at the protein level together with the activity of erbB signalling pathways following stimulation of CD44 by its ligand, hyaluronan (HA), or following stimulation with erbB ligands (TGFα, heregulin β1) after knockdown of CD44 expression using siRNA. Immunofluorescence microscopy was used to determine association between CD44 and erbB family members. Cell migration was assessed by seeding cells onto fibronectin-coated microporous inserts for 24 hours. CD44 expression levels were evaluated using paraffin-embedded tissue sections in a series of primary breast cancer tissues (n=103) from patients receiving endocrine therapy by immunohistochemical staining. Results: CD44 was overexpressed at the gene and protein level in TamR cells versus their endocrine sensitive counterparts. siRNA-mediated suppression of CD44 expression reduced basal EGFR/Her2 activity in TamR cells together with suppressing their migratory phenotype. siRNA-mediated inhibition of CD44 expression greatly attenuated the ability of erbB ligands to activate erbB receptors and inhibited TGFα and heregulin β1-induced migration. Conversely, stimulation of TamR, but not MCF7 cells, with HA promoted activation of erbB family members together with their downstream signalling intermediates ERK1/2, Src and AKT. Analysis of CD44 expression in clinical tissue revealed an association between elevated CD44 expression and a shortened response to endocrine therapy. Conclusions: These data demonstrate that overexpression of CD44 accompanies endocrine resistance where it enhances the sensitivity of these cells to factors (erbB ligands, HA) found within the tumour microenvironment. This may implicate CD44 in loss of endocrine response clinically and, therefore, provide a novel therapeutic target in endocrine resistant breast cancer. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3027.